Flow amplified steering systems are sometimes used on earthmoving and industrial type mobile vehicles. Such systems normally comprise a steering control or metering circuit and a directional control circuit. The main directional control circuit usually includes a pilot operated directional control valve which controls the flow of pressurized fluid from a source pump to and from one or more bidirectional steering motors or cylinders. The basic component of the steering circuit is normally a hand metering unit (HMU) which directs a metered volume of fluid to a selected end of the directional control valve in response to rotation of a steering wheel connected to the HMU. The degree of movement of the directional control valve is controlled by the degree of rotation of the steering wheel. If the steering wheel is rotated slowly, a low volume of pilot fluid will be directed to the directional control valve resulting in low spool movement. A faster rotation of the steering wheel directs a larger volume of pilot fluid to the directional control valve resulting in greater movement of the directional control valve. The volume of fluid directed to the steering motor is typically many times greater than the volume of pilot fluid directed to the directional control valve and is dependent upon the degree of the movement of the directional control valve and thus the degree and speed of rotation of the steering wheel.
One such system is disclosed in U.S. Pat. No. 4,914,913 to St. Germain et al. and issued on Apr. 10, 1990. In the reference patent, a load responsive flow amplified control system includes a directional control valve movable to an operating position to direct fluid from the operating fluid circuit through a first variable orifice to a steering motor and to direct control fluid from a control fluid circuit through a second variable orifice to be combined with the fluid going to the steering motor. The operating fluid circuit and the control fluid circuit are provided with fluid from a common variable displacement pump. The directional control valve is moved to an operating position by the control fluid acting thereon and is pressure compensated insofar as the flow of control fluid passing through the second variable orifice such that a substantially constant predetermined pressure differential exists across the second variable orifice. The rate of fluid flow in the pilot control circuit is determined by a steering control valve and is utilized for controlling the position of the directional control valve. The displacement of the variable displacement pump is responsive to the pressure condition in the pilot control circuit such that a substantially constant predetermined pressure differential exists between the variable displacement pump and the load pressure in the steering motor. The relative size of the variable orifices determines the amplification ratio between the pilot control circuit and the power operating circuit.
The variable displacement pump used as the primary source of hydraulic fluid in the load responsive flow amplified control system typically must be of substantial size to provide hydraulic fluid to the control system and other systems of the machine in which the control system is implemented. Additionally, backup sources still must be implemented to ensure that the machine does not lose steering control in the event of a failure of the primary pump. Such backup sources, which may be in the form of a backup pump or pressurized fluid accumulators, require additional space within the machine, and can increase the overall size of the machine or require the elimination of other features of the machine in the interest of conserving space and manufacturing costs. In view of this, opportunities exist for improving the space utilization and compactness of machines having load responsive flow amplified control systems.